FIG. 1 is a conventional touch screen. The touch screen comprises a display controller 130, a touch panel 150, a sensing circuit 155 and a liquid crystal display (LCD) panel 170. In general, the touch panel 150 is fabricated on the LCD panel 170. The display controller 130 receives a video signal and converts the video signal to a panel control signal transmitted to the LCD panel 170 so that the LCD panel 170 displays the image according to the panel control signal. When one touches the touch panel 150, the touch panel 150 generates a sensing signal to the sensing circuit 155, and the sensing circuit 155 outputs according to the sensing signal a position signal that represents a touch point where one touches the touch panel 150.
FIG. 2A is a diagram of the LCD panel. The LCD panel 170 is generally divided into two regions—a display region 112 and a non-display region 114. The display region 112 comprises a thin film transistor (TFT) array, and the non-display region 114 comprises a gate driver 120 and a source driver 125 for controlling transistors in the TFT array. The panel control signal outputted from the display controller 130 controls the gate driver 120 to generate a gate driving signal and the source driver 125 to generate a source driving signal. The panel control signal further comprises a common voltage signal Vcom for controlling the inversion of a liquid crystal on the LCD panel 170. The gate driving signal controls respective TFTs within the TFT array to whether turn on or turn off, and the source driving signal provides brightness data to a pixel. In FIG. 2A, in a portable electronic device application, the display controller 130 can be integrated with a timing controller (TCON), the gate driver 120 and the source driver 125.
The video signal comprises a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a red signal, a green signal and a blue signal. The time to display a scan line on the LCD panel 170 is a period of the horizontal synchronization signal Hsync, while the time to display a frame on the LCD panel 170 is a period of the vertical synchronization signal Vsync. That is, if the LCD panel 170 has M scan lines, the gate driver 120 can generate M gate driving signals, and one period of the vertical synchronization signal Vsync equals M periods of the horizontal synchronization signal Hsync. According to the horizontal synchronization signal Hsync, M gate driving signals can be turned on sequentially.
FIG. 2B is a diagram of the vertical synchronization signal Vsync, the horizontal synchronization signal Hsync, the common voltage signal Vcom and the gate driving signals. The period of the vertical synchronization signal Vsync begins from the start of the low level, and one period of the vertical synchronization signal Vsync equals a plurality of periods of the horizontal synchronization signal Hsync. According to the horizontal synchronization signal Hsync, a plurality of gate driving signals can be turned on sequentially, and a frequency of the common voltage signal Vcom is a half of a frequency of the horizontal synchronization signal Hsync. As mentioned, in the portable electronic device application, the display controller 130 can be integrated with the timing controller, the gate driver 120 and the source driver 125, with the common voltage signal Vcom being present in the integrated display controller 130.
During a vertical blanking interval (VBI), the red signal, the green signal and the blue signal do not output any data, while the common voltage signal Vcom also remains at the low level.
FIG. 3 is a diagram of a conventional capacitive touch panel. The capacitive touch panel comprises a first sensing layer 151, a second sensing layer 152 and a shielding layer 153. Generally, the first sensing layer 151 and the second sensing layer 152 respectively comprise a plurality of sensing components, each of which can be viewed as a capacitor.
When one touches the capacitive touch panel, an equivalent capacitance of the touch point is changed. By detecting the change in the equivalent capacitance of the touch point, the sensing circuit 155 can detect the actual position of the touch point and output a corresponding position signal. The shielding layer 153 is mainly for isolating the panel control signal from the sensing signal so that the sensing signal is not undesirably affected by noises from the panel control signal.
FIG. 4 is a diagram of a conventional capacitive touch sensing apparatus 40. The apparatus 40 comprises a protecting layer 420, a touch panel 450, a sensing protection layer 457 and a Vcom signal layer 480. The protecting layer 420 protects the capacitive touch sensing apparatus 40 from scratches that may be caused by touching. The sensing protection layer 457 protects the touch panel 450. The touch panel 450 comprises a first sensing layer 451, a second sensing layer 452 and a shielding layer 453. Generally, the first sensing layer 451 and the second sensing layer 452 comprise a plurality of sensing elements, and each sensing element can be viewed as a capacitor. Since sensing elements of the sensing layers 451 and 452 are usually capacitors, the touch panel 450 is additionally provided with the sensing protection layer 457 for protecting the touch panel 450 from deformation.
When one touches the capacitive touch panel, an equivalent capacitance of the touch point is changed. Hence, the capacitive touch sensing apparatus 40 can use such characteristic to detect the actual position of the touch point and output the position signal. Since signals in the conventional Vcom signal layer 480 are alternating current (AC) signals that are constantly transitive, significant noise interference imparted to the touch panel 450. To render shielding effects against the noise interferences, the shielding layer 453 is provided to isolate the Vcom signal layer 480 and the touch panel 450 from each other. However, helpful to reduce noise interference, the shielding layer 453 increases the manufacturing cost and also reduces transmittance of the touch panel 450. Further, during the manufacturing process of the touch panel 450, the shielding layer 453 needs to be adhered with the second sensing layer 452 and the sensing protection layer 457, and so the manufacturing cost increases from having to discard the entire touch panel 450 in the event of an adherence failure. Further still, due to the additionally provided shielding layer 453, the touch panel 450 includes not three layers but four layers, which compromises the transmittance of the capacitive touch sensing apparatus 40.
FIG. 5 is a diagram of the relation between a conventional source driving signal and a common voltage signal Vcom. The conventional touch sensing apparatus uses an AC Vcom signal. For example, the AC Vcom signal swings from −1V to 4V, and then a source swing of the source driving signal need an inversion signal of −1V to 4V to provide a voltage difference of 5V between the source driving signal and the AC Vcom signal; that is, the source driving signal provides a working voltage range of 5V.
To accommodate the AC Vcom signal adopted in the conventional touch sensing apparatus, the four-layered capacitive touch sensing apparatus is needed. As discussed above, the conventional four-layered capacitive touch sensing apparatus is not only costly but also has unsatisfactory transmittance. Therefore, there is a need for a capacitive touch sensing apparatus that overcomes the above shortcomings.